Question: We are currently planning a new drum system in which steel parts are to be alkaline galvanized and passivated. Unfortunately, our space is limited, which is why we have to save on tanks based on the "perfect condition". During the planning stage, we began discussing whether to dispense with electrolytic degreasing. After all, longer cycle times would allow us to spend more time in the boiling degreasing process. As the pickling process would be the last step of the pre-treatment, it would even be possible to save on decapitation. Our electroplating foreman is vehemently opposed to this, both internally and at the plant manufacturer. Tests in small series have shown that it makes no difference to appearance and adhesion whether we degrease electrolytically or not. What recommendation would you make?
Answer: If we interpret your comments correctly, we would first recommend giving the electroplating foreman a pay rise and giving him full responsibility for the project. He obviously not only has the necessary expertise, but also the necessary integrity to stand his ground despite the headwinds. To paraphrase Bertrand Russell: "Even if everyone agrees, everyone can be wrong."
General observation
The comparison is a little off, but electrolytic degreasing can in many cases be seen as a reserve parachute for a parachute jump. It is very rarely needed, but when it is, you are glad you have it.
Preliminary tests in small series can provide general information about feasibility, but say nothing about day-to-day practice. There are numerous reasons for this. Preliminary tests often show the ideal state. They use new chemicals, the tubs are in perfect condition and the starting material is often flawless compared to a large series. There is less dirt and imperfections on the surface and the parts have not been stored for long. And everyone involved in the trials is keen to achieve a perfect result, as there are high expectations.
Everyday practice is completely different. The goods have more oil, more impurities, possibly traces of corrosion and may have been stored for much longer. Pre-treatment is not like a new batch every day, the system will no longer be in the very best condition in a few years' time and experience has shown that pre-treatment control will also deteriorate. This will eventually become a problem even with a perfect system.
In addition, adhesion tests are often neglected when galvanizing steel. This means that problems can occur even after 6 to 12 months. You can imagine what it means in the automotive industry, for example, if a year's production is turned into a recall campaign. If it then transpires that the plant planning was carried out in contravention of all applicable standards and all specialist knowledge, the insurance cover expires faster than you can say "savings potential".
And to point out - quite independently of the case described - the need for good pre-treatment: Around 80% of all electroplating defects can still be traced back to incorrect or inadequate pretreatment. 50-60 % of defects are attributed to inadequate degreasing treatment.
Function and purpose of electrolytic degreasing
Electrolytic degreasing has a decisive advantage over chemical degreasing: loose, adhering particles are blown away by the formation of gas bubbles. During electrolysis, fine gas bubbles form on the surface of the material, which grow steadily and, once they reach a certain size, break off and rise to the surface. As the gas bubbles form between the (metallically conductive) workpiece and the (non-metallic, barely conductive) impurities, the dirt is blasted off. The gas bubbles consist of oxygen at the anode and hydrogen at the cathode.
In essence, the aim is no longer to remove coarse oils and fats, but to achieve ultra-fine cleaning. It is occasionally argued that this ultra-fine cleaning is not necessary for drum goods, as the surface would be given an additional cleaning effect by the mechanics. However, this is only true to a limited extent and only for parts with perfect geometry. Angled objects, internal threads or flat washers, on the other hand, have individual problem zones in which the mechanics are not effective or are even detrimental - for example the sticking of washers in the drum. This is where electrolytic treatment is particularly helpful in driving the parts apart.
The value of electrolytic degreasing was already clearly illustrated in [1] in 1964:
"In electrolytic degreasing, the purely chemical or colloidal chemical action of the alkaline cleaning solution is enhanced by the action of the electric current. For this reason, electrolytic degreasing is currently the best cleaning process. No other technical process provides metal surfaces of such a high degree of purity for galvanic purposes."
One could fall for the idea of replacing boiling-off degreasing with the much "better" electrolytic degreasing. But there are two problems with this:
This type of degreasing only works excellently if there are only minor impurities left on the surface. The mechanism described above only works inadequately if the surface is electrically insulated by grease, oil, corrosion products or corrosion protection products.
In addition, there is another important task of electrolytic degreasing: the removal of pickling deposits. This means that this process step must follow the pickling process.
No need for decapping?
We also strongly advise against dispensing with decapitation, possibly on the grounds that immersion from one alkaline solution into another alkaline solution is required anyway.
The purpose of decapitation is to chemically remove surface films that have formed during the process. These are usually microscopically thin, invisible layers that can severely impair the adhesive strength of the subsequent electrodeposited coating or, depending on the type and thickness, even prevent adhesion.
Surface films can be
- Oxides
- oxide hydrates
- hydroxides
- silicates
- borates
- carbonates
- sulphides
- poorly soluble reaction products of the chemicals used with components of the substrate material.
It is therefore not about neutralizing the surface, as is often assumed. The French origin of the word de = remove and cape = cloak explains the aim quite well.
Savings
We also understand your dilemma. We would therefore like to offer a few suggestions below, although this is difficult without knowing the details. We assume that you have already considered the possibilities of several streets.
Smaller tubs
A few meters could be saved by using smaller or narrower tubs. The problem of a lower throughput could possibly be compensated for by double drums.
Reduction of sinks
Triple cascades can sometimes be replaced in the pre-treatment stage by a pedestal sink / flow sink. Either the rinsing quality or the load on the circulation water suffers as a result, but this can be calculated in advance to determine whether it will be a problem.
Combining sinks
Cascades, pedestal sinks and flow sinks can be combined. For example, the rinsing of boiling and electro-degreasing. In the case of cascades, degreasing and pickling rinses can be used together. The associated neutralization also has positive effects, but of course there is a double load on the rinsing volume.
B. C.
Literature
[1] R. Bilfinger; W. Burkhardt: In Dettner/Elze: Handbuch der Galvanotechnik Vol. I, 994-995, Carl-Hanser-Verlag
[2] B. Gaida; B. Andreas; K. Aßmann: Technologie der Galvanotechnik, Eugen G. Leuze Verlag
